1. Field of the Invention
The present invention relates to integrated circuit fabrication, and more particularly to the reduction of voiding in Al metal lines.
2. Description of the Related Art
Metallization lines in advanced integrated circuits are often fabricated from sputter-deposited Al/Ti bilayers. In particular, sputtered Ti is widely used in integrated circuits as an underlayer film for Al-based multilayer metallizations. The Al/Ti bilayer has improved electromigration resistance as compared to a single layer Al metallization. However, Ti and Al react at temperatures above 350.degree. C. to form TiAl.sub.3, producing a 5.9% volumetric contraction. In a typical interconnect processing sequence, the Al/Ti alloy metallization stack is deposited at temperatures &lt;300.degree. C. followed by lithographic patterning and etching. Dielectric passivation deposition occurs at elevated temperatures &gt;350.degree. C. Upon cooling, the metal line cross-section, which is rigidly attached to the encapsulating material, is subject to a large tensile hydrostatic stress state since the Al has a larger coefficient of thermal expansion than the passivation. Subsequent thermal cycles often reach temperatures greater than 400.degree. C., leading to further TiAl.sub.3 formation and volume contraction. Strain induced by the volumetric contraction of TiAl.sub.3 leads to increases in void formation in the Al metal line because voiding is a tensile hydrostatic stress relaxation mechanism. See Besser, P. R., Sanchez, J. e. Jr., and Alvis, R., ., The Effect of Si on TiAl3 Formation in Ti/Al Alloy Bilayers, Proceedings of the 1994 MRS Fall Meeting, Vol. 355, p. 631, Materials Research Society, Pittsburg, Pa. (1995).
The reaction rate for the Ti+3Al=&gt;TiAl.sub.3 reaction has been shown to be a function of the concentration of alloying elements such as Cu and Si in the Al. As a result, Al:Si alloys have been used to reduce TiAl.sub.3 formation. See Besser et al. Si has also been used in NMOS integrated circuits as an alloying element in Al for reduction of junction spiking in simple Al-to-Si ohmic contact structures. However, in both cases deposited films containing both Al and Si (typically, .about.1% wt Si), exhibit Si precipitation problems. During the cooling cycle of a thermal anneal, the solid solubility of Si in Al decreases with decreasing temperature. The Al thus becomes supersaturated with Si, which causes nucleation and growth of Si precipitates out of the Al:Si solution. Si precipitates formed within the Al interconnect lines can increase the susceptibility of the lines to electromigration failure. In narrow Al lines, the precipitates can be large enough to obstruct a large fraction of the cross-sectional area of the metal line. In particular, the size of the Si precipitates in Al: 1% Si alloys can range from about 0.4-1.5 .mu.m, depending on how slowly the films are cooled. At locations where such precipitates are formed, a large flux-divergence in the current is produced. This flux divergence, in turn, can lead to early failure of the conductor by an electromigration-induced open circuit. See generally, Wolf, Silicon Processing for the VLSI Era: Volume 2-Process Integration, .sctn. 3.4.4, Lattice Press, 1990.